A cold front extending from a surface low over Saskatchewan, combined with a strong low pressure trough over northeast Montana amid a very unstable atmosphere to produce numerous thunderstorms, some of which became severe. Later in the evening, some of these storms merged together, produced strong winds as they departed Montana.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS GGW wrote it up.
A cold front extending from a surface low over Saskatchewan, combined with a strong low pressure trough over northeast Montana amid a very unstable atmosphere to produce numerous thunderstorms, some of which became severe. Later in the evening, some of these storms merged together, produced strong winds as they departed Montana. In addition, heavy rain, caused a few roads to wash out.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Glasgow, MT wrote its last area forecast discussion before the first event at May 26, 2:39 PM MST, 3 hours 12 minutes earlier.
Area Forecast Discussion National Weather Service Glasgow MT 339 PM MDT Sat May 26 2018 .DISCUSSION... The stage is set and the atmosphere is primed for the development of some strong to severe thunderstorms beginning now and lasting through tonight. Favorable shear profile, warm moist air at the surface on a southeasterly flow, and an advancing cold front from the southwest are all contributing to the threat for these severe thunderstorms. Most of our CWA has been outlined for a slight risk area from SPC all day long. Last mesoscale discussion calls for 60% chance of a severe thunderstorm watch being issued for this area. Even now, isolated thunderstorms are breaking out over the higher terrain of central Montana. Expect coverage and intensity to increase with time this evening and tonight. Sunday morning, as the last lingering storms slide eastward into North Dakota and the cold front advances further, temperatures will cool a bit. Sunday afternoon and evening, the front will not have completely advanced all the way through the CWA. This will leave an unstable atmosphere over our SE zones along the Yellowstone River valley. Would not be surprised to see another strong to marginally severe thunderstorms down there through the evening and late night hours. Monday through Tuesday, the large upper-level low pressure system stalls over the Great Basin region, keeping a generally unsettled weather pattern over NE Montana. Wednesday, and brief calm period takes over, thanks to a weak upper-level ridge over the area. Thursday and beyond, another cold front will be the focus for more thunderstorm development over the region. BMickelson && .AVIATION... FLIGHT CAT: VFR to begin, becoming IFR or worse in severe thunderstorms after 22Z. DISCUSSION: Thunderstorms, with some becoming severe are expected to impact the terminals in some way later this afternoon into the overnight hours. Thunder should leave the area between 06Z to 09Z but may linger just a bit longer near KSDY. Expect threat of large hail and damaging winds. Some MVFR/IFR conditions are expected with/near these impacts, especially if a TAF site is directly under one of these thunderstorms. WIND: Southeast at 10 to 15 kts in the afternoon. Erratic and damaging winds in excess of 55 mph are possible near the stronger storms between until 09Z overnight tonight. Winds will switch around from the north behind thunderstorms tonight. BMickelson && .GLASGOW WATCHES/WARNINGS/ADVISORIES...NONE. && weather.gov/glasgow
Special Weather Statement
National Weather Service Glasgow MT
634 PM MDT SAT MAY 26 2018
MTZ017-270115-
Central and Southern Valley MT-
634 PM MDT SAT MAY 26 2018
...SIGNIFICANT WEATHER ADVISORY FOR NORTHERN GARFIELD AND SOUTH
CENTRAL VALLEY COUNTIES UNTIL 715 PM MDT...
At 633 PM MDT, Doppler radar was tracking a strong thunderstorm 11
miles north of Hell Creek Rec Area, or 30 miles southwest of Glasgow,
moving northeast at 20 mph.
Dime size hail and wind gusts up to 50 mph will be possible with this
storm.
Locations impacted include...
The Pines Rec Area.
A severe thunderstorm watch remains in effect until 100 AM MDT for
northeastern Montana.
LAT...LON 4794 10651 4773 10677 4774 10682 4773 10687
4769 10687 4768 10688 4767 10692 4768 10699
4766 10701 4766 10707 4767 10711 4767 10716
4766 10718 4766 10721 4806 10681
TIME...MOT...LOC 0033Z 223DEG 16KT 4779 10689
MotticeSpecial Weather Statement National Weather Service Glasgow MT 636 PM MDT SAT MAY 26 2018 MTZ022-023-270115- Garfield MT-McCone MT- 636 PM MDT SAT MAY 26 2018 ...SIGNIFICANT WEATHER ADVISORY FOR EAST CENTRAL GARFIELD AND WEST CENTRAL MCCONE COUNTIES UNTIL 715 PM MDT... At 636 PM MDT, Doppler radar was tracking a strong thunderstorm near Nelson Creek Rec Area, or 31 miles northeast of Jordan, moving north at 10 mph. Nickel size hail and wind gusts of 50 to 55 mph will be possible with this storm. Locations impacted include... Nelson Creek Rec Area and Rock Creek Rec Area. PRECAUTIONARY/PREPAREDNESS ACTIONS... This storm may intensify, so be certain to monitor local radio stations and available television stations for additional information and possible warnings from the National Weather Service. && A severe thunderstorm watch remains in effect until 100 AM MDT for northeastern Montana. LAT...LON 4744 10621 4748 10646 4778 10641 4769 10587 TIME...MOT...LOC 0036Z 191DEG 10KT 4752 10630 Mottice
The Glasgow radar (KGGW), 79 km away, was scanning. The volume nearest the first event began at 00:50 UTC.
14 hail, 4 thunderstorm wind, 2 flash flood. Local standard time, as filed.
Tennis ball size hail was reported via social media. The report included photographic evidence.
Hail up to two inches in diameter was reported via social media. The report included photographic evidence.
A report of golf ball size hail was received via social media. The report included photographic evidence.
A report of tennis ball size hail was received via social media. Most of the hail that fell was hen egg size (two inches in diameter), with a few hailstones the size of tennis balls. The report included photographic evidence.
A trained spotter reported a lot of nickel size hail with a few hail stones the size of quarters.
NWS employee reported hail up to the size of ping pong balls.
Public reported hail at least the size of quarters.
Public reported ping pong ball size hail.
Via social media, public reported golf ball size hail.
Public reported ping pong ball size hail and heavy rain.
Trained spotter reported two large doors on a 150-foot quonset blown in and part of the fabric roof torn off. Time estimate from radar imagery.
Trained spotter reported hail up to the size of quarters and heavy rain.
Trained spotter reported ping pong ball size hail and heavy rain.
Trained spotter reported hail up to the size of quarters and heavy rain.
Sioux Pass MT-16 DOT site measured a 64 mph wind gust.
ASOS site at the Sidney airport measured a 66 mph wind gust.
Public reported hail up to the size of ping pong balls and heavy rain.
Via social media, public reported tree limbs broken and damage to a field.
Public reported roads washed out leading into Rock Creek State Park.
Trained spotter reported water around 2 to 3 feet deep flowing over multiple roads.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2018Every state in 2018This day in weather historyWarnings in Montana, May 2018
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 757848, 757849, 757850, 757851, 757852, 757939, 757925, 757930, 757915, 757918, 757931, 757919, 757926, 757920, 757932, 757921, 757927, 757937, 757928, 757933.